Shifting elements for torque transmission, for example in the form of shifting clutches or claws, have long been known in geared mechanical transmissions such as countershaft transmissions. Countershaft transmissions are usually made with gearwheels, so-termed loose wheels, mounted rotatably, on a shaft, which mesh with gearwheels connected in a rotationally fixed manner to other shafts so that they form so-termed gear pairs. By alternately connecting the gearwheels mounted rotatably on the shaft to the shaft in a rotationally fixed manner, the gear pairs can be connected into the force flow of a countershaft transmission by means of appropriate shifting elements such as synchronizers, claws or frictional elements in order to obtain various gear steps. As a rule this requires an actuating element in the form of a manual actuator, an electric actuator or a piston/shifting-cylinder arrangement that can be actuated hydraulically or pneumatically. In such piston/shifting-cylinder arrangements a piston is arranged and able to move axially in a shifting cylinder. The piston can move actively to at least one shift position and in its thrust direction is directly or indirectly connected fixed to the shifting element. In this case the actuating element can actuate the shifting elements mechanically, hydraulically, pneumatically or magnetically in relation to the shaft on which the rotatably mounted gearwheels are arranged.
A shifting element usually belongs to a shifting group of a geared mechanical transmission. In such a case a shifting group is preferably formed of two rotatably mounted gearwheels arranged next to one another in the transmission, which can alternately be engaged in the torque transmission by means of a shifting element. As is known, for example from the document DE 197 56 639 A1 by the present applicant, in such cases for each shifting group of a geared transmission, one shifting element with a cylinder that acts on both sides, having a piston which is moved by a fluid, can be used.
Depending on the design, it can also be necessary to maintain a central position in a shifting group, this as a rule being the neutral position. In the central or neutral position it is important that no part of the shifting element engages with any loose wheel. Thus, for example the central position constitutes the passive position of the actuating element. In that case the actuating element is held in the central position for example by springs, and has to be pushed actively to the other two shifts positions. For this, in pneumatic or hydraulic control systems two working lines and two piston faces are needed. Without such holding in the central position by spring action there is a risk that the central position will not be able to be approached, set and maintained exactly so that parts of a shifting element are at least partially engaged with a loose wheel.
The problem of a safe approach to the central position can also be solved by mechanical systems without the central position being held by springs. One solution is described, for example, in the document DE 40 38 170 A1. A shifting cylinder is considered, which has a two-sided controllable piston coupled to a shifting element, the piston being acted upon with a pressure medium by way of control elements such as displacement valves. The piston consists of two double pistons that act in opposition, which can move within a common cylinder housing, such that in the cylinder housing a respective pressure space with a pressure medium connection is associated with each of the two double pistons. The first piston of each double piston is rigidly connected to move with a shifting element and the second piston of each double piston is connected to a first piston so that it can move freely. When the pressure is equalized on both sides, the two double pistons are held in the neutral position with their freely movable, second pistons against a stop in the cylinder housing, and in each case when acted upon alternatively by pressure on one side or the other, they are pushed out of the neutral position to the shifting position, in such manner that the unpressurized double piston with its two pistons has its shifting displacement imparted to it by the pressurized double piston.
However, for example due to lack of fitting space or even by virtue of a component identity approach or platform concept, it can be in part not possible, or only so with difficulty, to realize a shifting cylinder that can be actuated to both sides with two working lines and with a corresponding control system.